The present invention relates to a comparator circuit and method. Such circuits and methods are, of course, already known. However, the known circuits and methods exhibit a number of disadvantages and the object of the present invention is to provide an improved comparator circuit and method.
A very substantial difficulty experienced in transistor implemented comparator circuits is the variation in threshold voltage which occurs between different transistors. This problem was encountered in the 1970s and techniques were devised to mitigate the effect of the non-uniformity of the threshold voltage. At that time, of course, the transistor fabrication was nMOS and subsequent improvements in the fabrication process for single crystal devices was such that no further attention was paid to the threshold variation problem. Recently this position has changed, particularly as a consequence of tie reduction in size of nMOS devices to the sub-micron level.
An example of a recent technique to compensate for threshold voltage variation in a single crystal transistor implemented comparator circuit is disclosed in U.S. Pat. No. 5,949,270 issued on Sep. 7, 1999. As disclosed in that document, a capacitor is connected between the gate of a transistor that is an object of threshold voltage compensation and an input terminal. A first switching device is connected between a current source connected to one term of the transistor and the gate of the transistor. A second switching device is connected between the input terminal and a terminal to which a reference voltage is applied. The first switching device is turned ON so that the transistor is diode-connected, The second switching device is turned ON, thus applying the reference voltage to the input terminal. A reference voltage is applied to a current inflow terminal connected to another terminal of the transistor. After charge dependent on the threshold voltage of the transistor is accumulated in the capacitor, the first switching device is turned OFF. With this control, a difference in threshold voltages between adjoining transistors can be compensated for.
Thin film transistors (TFTs), which are formed using a polycrystalline semiconductor film, such as polycrystalline silicon, are well known. Such polysilicon TFTs arc considered to have several advantages over MOS devices formed with a single crystal semiconductor. Principally, TFTs can be produced relatively inexpensively since the fabrication process avoids the constraints of producing a sufficiently large single crystal silicon substrate with a satisfactorily low level of impurities. Moreover, because the polycrystalline semiconductor film can be produced on any suitable supporting substrate, such as glass sheet, the size constraints necessitated by the production of a single crystal substrate are obviated, so that large numbers of TFTs, and hence a large number of circuits, can be produced on a relatively inexpensive single support substrate.
However, polysilicon TFTs have the significant drawback in that they have widely varying threshold voltages, even when manufactured in the same production batch and using the same polysilicon film. The threshold voltage is effectively the voltage which must be applied to the gate electrode of the TFT to enable a current to flow in the channel region between the source and drain regions and so determine the ON-condition of the TFT. In the fabrication process for the polysilicon film it is difficult to guarantee continuity of individual crystal sizes and, furthermore, there are also variations in the purity of the film. Thus, the polysilicon film material varies between the TFTs fabricated from a common polysilicon film and it is this film material which determines the threshold voltage of the TFTs. Hence, polysilicon TFTs exhibit a far greater variation in threshold voltage in comparison to MOS single crystal transistors. As a consequence of this variation in threshold voltage. TFTs have not been considered for many circuit applications, and in particular for those applications where consistency of transistor threshold voltage is of paramount importance, such as comparator circuits. This is because for such circuits to be of practical use in comparing the levels of signals applied to the circuit input terminals, a small difference occurring between the voltage level applied to one input terminal and the voltage level applied to another input terminal of the comparator is required to produce a large change in the voltage at the output terminal. If there is a significant difference between the threshold voltage of one transistor used to detect these levels and the threshold voltage of another transistor used to detect the levels, the difference between the voltages applied to the input terminals must exceed the difference in the threshold voltages between the transistors for any voltage to be provided at the output terminal. TFTs exhibit this variation in the threshold voltage which has severely hindered their use in comparator circuits.
Moreover, for TFTs, the current/voltage characteristic varies with the width/length ratio of the channel region. Additionally, for any channel region width/length ratio, the operational characteristic differs significantly between P and N channel devices. For example, for any change in voltage applied to the device, an N channel TFT will exhibit a much shaper rise in output current than for a P-channel TFT.
Several concerns also exist with the threshold compensation circuit as described in U.S. Pat. No. 5,949,270, such that it would not reliably function if implemented using TFTs.
The first switching device described in U.S. Pat. No. 5,949,270 is a MOS transistor connected as a diode which, in essence, acts as a load resistor for the second switching device. As it is a transistor connected to operate as a diode it will exhibit a non-linear impedance characteristic.
For comparator circuits, a prime requirement is to make the swing in the voltage at the output terminal as large as possible for any change between the voltages at the input terminals.
From FIG. 1, which shows the transfer characteristic of an inverter circuit implemented using TFTs, it can be seen that if such a circuit is used as a comparator it is necessary to operate the TFTs on the knee portions of the characteristic so as to ensure that there is a large swing in the output voltage for a small change in the input voltage. The non-linear characteristic of the first switching device of U.S. Pat. No. 5,949,270 coupled as a diode would not enable this to be achieved reliably in practice.
Furthermore, the circuit described in U.S. Pat. No. 5,949,270 is implemented using a pair of N-channel switching devices. Therefore, when the circuit is in operation both devices are ON at all times, creating high power consumption. Hence, it is not a straightforward step to implement the circuit shown in U.S. Pat. No. 5,949,270 using TFTs, and in particular using complementary TFTs so as to minimise power consumption in use of the circuit.
According to a first aspect of the present invention there is provided a comparator circuit comprising a pair of complementary thin film transistors serially coupled to provide an inverter and two capacitors, one side of each capacitor being operably coupled to a respective input terminal and the other side of each capacitor being connected in common to a node coupled to the gates of the transistors, and further comprising a switch coupled to the node, which is arranged to provide a voltage from the point of series connection of the transistors for storage in the capacitors to act as a bias voltage for voltages applied to the input terminals and thereby compensate any threshold voltage variation between the thin film transistors.
In addition to the differences in circuit arrangement between the first aspect of the present invention and the circuit described in U.S. Pat. No. 5,949,270, it will also be appreciated that the present invention uses polycrystalline thin film transistors rather than the single crystal transistors used in all of the prior art discussed above.
Preferably the comparator circuit according to the present invention comprises two further switches each selectively connecting a respective said other side of said capacitors between ground and the said respective input. Beneficially the switches are implemented by transistors. Very desirably all of the transistors are CMOS TFT transistors.
It is preferred that the comparator circuit of the present invention further comprises an output stage with said output stage being selectively isolated or connected to said transistor pair by an output stage switch. Furthermore, it is preferred that a buffer is connected between the output stage switch and the said transistor pair.
According to a second aspect of the present invention there is provided a method of comparing two input voltages by: providing a comparator having a pair of complementary thin film transistors serially coupled to provide an inverter, and two capacitors, operably coupling one side of each capacitor to a respective input terminal and connecting in common the other side of each capacitor to a node coupled to the gates of the transistors, and further providing a switch coupled to the node, closing the switch to transfer a bias voltage to the node, storing the bias voltage on the capacitors so as to compensate any threshold voltage variation between the thin film transistors, opening the switch, applying each input voltage to a respective one of the capacitors, and detecting any resultant change in the bias voltage.